Creating a custom permanent property in Blender 2.70 python script to access in GUI Panel
Since your example panel is in the object context I assume you want to use the data on objects.

If you want to store and retrieve properties on a per object basis you can append a property to objects like this:

bpy.types.Object.obj_property = bpy.props.FloatProperty(name = "ObjectProperty")


and access the value in the UI like this:

row.prop(context.object, "obj_property")


Or if you need your property for all objects in a scene you could store it inside the scene like this:

bpy.types.Scene.scn_property = bpy.props.FloatProperty(name = "SceneProperty")


and access the value in the UI like this:

row.prop(context.scene, "scn_property")


To access the property values in your script you can also use the bpy.data module.

Note that operators and panels are conceptually different. Panels are not supposed to store properties or change them on their own. They are just for user access to internal data and properties. Operators on the other hand have builtin access to properties and are better suited to manipulate data. Note also that properties are automatically stored in the .blend file while saving. If you want to share data between scenes you could use other ID blocks like Text or use a simple operator to copy data between scenes e.g. like this:

import bpy

class UIPanel(bpy.types.Panel):
  bl_label = "My Panel"
  bl_space_type = "PROPERTIES"
  bl_region_type = "WINDOW"
  bl_context = "object"
  bpy.types.Scene.scn_property = bpy.props.FloatProperty(name = "SceneProperty")

  def draw(self, context):
    self.layout.operator("prop.propcopy", text='Update Scenes')
    self.layout.prop(context.scene,"scn_property")

class OBJECT_OT_PropCopyButton(bpy.types.Operator):
  bl_idname = "prop.propcopy"
  bl_label = "Propcopy"

  def execute(self, context):
    sc0 = context.scene
    for sc in bpy.data.scenes:
        sc.scn_property = sc0.scn_property
    return{'FINISHED'}

bpy.utils.register_module(__name__)